In most current wireless communication systems, nodes in the network may be configured to operate based on a single transmit and a single receive antenna. However, for many current wireless systems, the use of multiple transmit and/or receive antennas may result in an improved overall system performance. These multi-antenna configurations, also known as smart antenna techniques, may be utilized to reduce the negative effects of multipath and/or signal interference may have on signal reception. Existing systems and/or systems which are being currently deployed, for example, code division multiple access (CDMA) based systems, time division multiple access (TDMA) based systems, wireless local area network (WLAN) systems, and orthogonal frequency division multiplexing (OFDM) based systems, such as IEEE 802.11a/g, may benefit from configurations based on multiple transmit and/or receive antennas. It is anticipated that smart antenna techniques may be increasingly utilized both in connection with the deployment of base station infrastructure and mobile subscriber units in cellular systems to address the increasing capacity demands being placed on those systems. These demands arise, in part, from the shift underway from current voice-based services to next-generation wireless multimedia services that provide integrated voice, video, and data transmission.
The utilization of multiple transmit and/or receive antennas is designed to introduce a diversity gain and to suppress interference generated within the signal reception process. Such diversity gains improve system performance by increasing received signal-to-noise ratio, by providing more robustness against signal interference, and/or by permitting greater frequency reuse for higher capacity. In communication systems that incorporate multi-antenna receivers, a set of M receive antennas may be utilized to null the effect of M−1 interferers. Accordingly, N signals may be simultaneously transmitted in the same bandwidth using N transmit antennas, with the transmitted signal then being separated into N respective signals by way of a set of N antennas deployed at the receiver.
This type of systems may be referred to as multiple-input multiple-output (MIMO) systems. One attractive aspect of multi-antenna systems, in particular MIMOs, is the significant increase in system capacity which may be achieved by utilizing these transmission configurations. For a fixed overall transmitted power the capacity offered by a MIMO configuration may scale with the increased signal-to-noise ratio (SNR). For example, in the case of fading multipath channels, a MIMO configuration may increase system capacity by nearly M additional bits/cycle for each 3-dB increase in SNR.
However, the widespread deployment of multi-antenna systems in wireless communications, particularly in wireless handset devices, has been limited by the increased cost that results from the increased size, complexity, and power consumption. Providing a separate RF chain for each transmit and receive antenna is a direct factor in the increased the cost of multi-antenna systems. Each RF chain generally comprises a low noise amplifier (LNA), a filter, a downconverter, and an analog-to-digital converter (A/D). In certain existing single-antenna wireless receivers, the single required RF chain may account for over 30% of the receiver's total cost. It is therefore apparent that as the number of transmit and receive antennas increases, the system complexity, power consumption, and overall cost may increase.
Furthermore, multi-path propagation in band-limited time dispersive channels may cause inter-symbol interference (ISI), which has been recognized as a major obstacle in achieving increased digital transmission rates with the required accuracy. ISI may occur when the transmitted pulses are smeared out so that pulses that correspond to different symbols are not discernable or separable. Meanwhile, data received from a desired user may be disturbed by other transmitters, due to imperfections in the multiple access scheme, giving rise to inter-carrier interference (ICI). For a reliable digital transmission system, it is desirable to reduce the effects of ISI and ICI.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of ordinary skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.